Advancement in optical communication technique enabling large volume data communication at high speed is becoming significant in recent years, and the optical communication network thereof is also continuously expanding. The optical communication technique is used in long range communications traversing the country and in medium range communications within a region, but is also used in short communication range such as optical signal transmission etc. inside the device or between devices.
Wiring must be performed so as to thread through a narrow gap between components since various components are closely arranged in portable devices, compact devices and the like. Thus, a flexible print wiring substrate is being widely used for the electrical wiring. Similarly, a flexible film waveguide is desired for transmitting the optical signal at a short distance such as inside the device, and between devices. In particular, when wiring the waveguide inside the portable compact device, wiring is often performed so as to lie on the surface of the part to reduce space, and thus a polymer film waveguide that can be bent with a small curvature radius is desired.
The material that has high flexural performance and that is easily deformable includes elastomer. Elastomer is a generic term for polymer material having rubber elasticity at normal temperature, and generally refers to materials having low flexural modulus such as rubber. The reason why the low flexural modulus of the elastomer is low will now be described. Elastomer has low glass transition temperature, and the polymer molecules thereof perform Brownian motion at room temperature. That is, elastomer has fluidity. Although the polymer molecules constituting the elastomer exhibit fluidity since the molecule chains thereof are chemically cross-linked, such fluidity is partial. Therefore, the elastomer has a rubber property of being easily bendable although it is a solid.
The elastomer is obtained by curing the monomer or the oligomer, which is the precursor thereof, by irradiating energy. Most elastomer have the monomer or the oligomer bonded and cross linked by hydrogen bonding of hydrophilic groups, and most precursors thereof contain hydrophilic groups in the molecules. The mixture of the precursor has lower fluidity since the hydrophilic groups hydrogen bond and exhibit the property of high viscosity. When the mixture of the precursor thereof is cured by energy irradiation, it becomes an elastomer of rubber form having small bending elasticity.
Therefore, a film waveguide that can be bent with a small curvature radius is assumed to be manufactured using the elastomer. FIGS. 1(a) to 1(g) are schematic cross sectional views describing the method of manufacturing the conventionally proposed film waveguide. In such manufacturing method, the clad material 12 is first dropped onto a substrate 11, as shown in FIG. 1(a). The clad material 12 is monomer or oligomer, which is the precursor of the elastomer having low refraction index. Subsequently, the clad material 12 on the substrate 11 is spread thinly by a spin coater, and the clad material 12 is cured by energy irradiation to obtain the clad layer 13, as shown in FIG. 1(b). Thereafter, the surface of a lower clad layer 13 is patterned and a concave groove 14 is formed, as shown in FIG. 1(c), and a core material 15 having a refraction index higher than the lower clad layer 13 is filled into the concave groove 14, as shown in FIG. 1(d). The core material 15 is monomer or oligomer, which is the precursor of the polymer having a refraction index higher than the lower clad layer 13. When the energy is irradiated onto the core material 15, the core material 15 is cured, and the core 16 having a refraction index higher than the lower clad layer 13 is formed in the concave groove 14, as shown in FIG. 1(e). As shown in FIG. 1(f), the clad material 12 (precursor of elastomer) same as the lower clad layer 13 is dropped onto the lower clad layer 13 and the core 16, and then thinly spread by spin coating, and thereafter, the clad material 12 is cured by irradiating energy, and the upper clad layer 17 made up of clad material 12 is formed, whereby the film waveguide 18 is manufactured, as shown in FIG. 1(g).
The elastomer having the low flexural modulus of smaller than or equal to 1,000 MPa is desirably used for the film waveguide that can be bent at a small curvature radius. However, since the viscosity becomes high or about 1,000 cP (=10 Pa·s) in the precursor of such elastomer, if the relevant elastomer is used for the upper clad layer 17 and for the lower clad layer 13, the film thickness of the clad layer obtained by spin coating can only be thinned to about 600 μm at the most, and thus the thin film waveguide having a thickness of smaller than or equal to 1,200 μm is difficult to obtain. Thus, even if the elastomer having the low flexural modulus of smaller than or equal to 1,000 MPa is used, it cannot be bent at small curvature radius due to its thickness.
The film thickness of the clad layer obtained by spin coating can be thinned by reducing the viscosity of the precursor of the elastomer. However, if the viscosity of the precursor of the elastomer is reduced, the low flexural modulus of the elastomer (clad layer) after curing increases, and consequently, the film waveguide that can be bent at small curvature radius becomes difficult to obtain.
Therefore, in the conventional method of manufacturing the film waveguide using the spin coating method, the film thickness of the clad layer cannot be thinned if the elastomer which viscosity of the precursor is high is used, and the low flexural modulus of the clad layer increases if the elastomer which viscosity of the precursor is low is used. In either case, the film waveguide that can be bent at a small curvature radius or the curvature radius of about a few mm cannot be manufactured.
When attempting to obtain the thin film waveguide 18 in such manufacturing method, there is only the method of thinning the lower clad layer 13 or the upper clad layer 17 through polishing etc. after curing the lower clad layer 13 or after curing the upper clad layer 17, but such method requires a great number of steps to obtain the thin film waveguide 18 and thus is disadvantageous in terms of productivity.
Patent Document 1 discloses the waveguide that uses urethane ultraviolet curable resin as the core material, which waveguide has a thickness of 1.5 mm only with one clad substrate and thus cannot be expected to be bent at small curvature radius.
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-90532